1. Field of the Invention
This invention relates to a wheel slip controlling system for controlling a frictional force between a tire of a driven wheel and the road surface not only during braking but also during acceleration of a car, and more particularly to a wheel slip controlling system for controlling rotation of a driven wheel by a brake mechanism as well as for controlling the output power of an internal combustion engine.
2. Prior Art
A prior art traction controlling system controls rotation of a wheel during acceleration of a car to provide a maximum frictional force between a tire of a wheel and the road surface so as to improve the stability and acceleration. In this kind of traction controlling system, slipping of a driven wheel is detected and the power of the internal combustion engine is controlled by controlling the ignition timing or fuel injection amount so as to control the rotation of the driven wheel when the degree of slipping is above a predetermined value. However, there are problems with the wheel slip controlling system mentioned above, and this system is regarded as insufficient.
Namely, the conventional wheel slip controlling systems aim at the prevention of slip during acceleration by reducing the fuel injection amount or setting a time lag against a change in the ignition timing so as to control the output of the internal combustion engine. In this case, however, the control capacity is limited, preventive measures against abnormal vibration may have to be taken, or the engine may stop due to abrupt changes in its operating conditions. When the intake air amount is controlled, on the other hand, the ignition timing and the fuel injection amount, etc. are decided in accordance with it. Therefore, the output power of the internal combustion engine can be controlled smoothly to improve the vehicle's drivability. However, if the position of the throttle valve interlocked to the accelerator pedal is controlled by a pedal depressed by the driver, some problems may occur such as kickback to the driver or instability in case of failure of control parts of the throttle valve.
Another problem is that the rotation of driven wheels cannot be controlled immediately due to slow responsiveness of the vehicle when the output power of the internal combustion engine is used for controlling the rotation of the driven wheels. Accordingly, it is desirable to control the rotation of driven wheels directly by using the brake system mounted on the car when the slip of driven wheels occurs during acceleration, but a special brake system will be required to control instantaneously the rotation of driven wheels by using the brake system only. Namely, if the driving force is rather great such as, for example, when the gear position of the transmission is set at the first speed, a special brake system which produces greater braking force will be necessary to give the corresponding braking force against the driving force. The conventional brake system may not work well enough. Also, a pressure source will be required for the purpose of increasing hydraulic pressure supplied to a braking cylinder equipped for the driven wheels of the control system for the car. Therefore, other devices such as an oil pump, etc. must be added to the conventional car for this purpose.